Electroless deposition of palladium in the manufacture of printed circuit boards, IC substrates and the like as well as metallization of semiconductor wafers is an established technique. The palladium layers are used for example as barrier layers and/or wire-bondable and solderable finishes.
Electroless palladium plating bath compositions comprising a source for palladium ions, a nitrogenated complexing agent and a reducing agent selected from formic acid and derivatives thereof are disclosed in U.S. Pat. No. 5,882,736. Such electroless palladium plating bath compositions are suited to deposit pure palladium in contrast to plating bath compositions containing hypophosphite as reducing agent which result in palladium-phosphorous alloy layers.
Patent application WO 2006/065221 A1 discloses an autocatalytic electroless plating bath for plating a metal, i.a., palladium, comprising a surfactant, i.a., nonylphenol ethoxylate, and a reducing agent, i.a., hydrazine or formaldehyde. When operated above the cloud point, the bath containing the surfactant leads to controlled deposition of the metal, reduced decomposition of the bath and provides high plating speed at very low concentrations of metal.
Patent application DD 222 346 A1 discloses a solution for autocatalytic deposition of palladium comprising a palladium compound, a reducing agent, complexing agents, stabilizers and nonylphenol as a surfactant. Addition of nitroso naphthols results in improved stability of the solution.
U.S. Pat. No. 4,424,241 describes an electroless plating solution comprising palladium, organic ligands and reducing agents, i.a. formaldehyde and formic acid. Certain additives, i.a. phenolphthalein, may be added to improve the appearance and properties of the plated palladium.
Although much of the prior art literature teaches palladium plating bath compositions, the plating rates obtained therewith cannot satisfy the current need for steadily increasing plating rates that are required in order to achieve economic manufacturing.
Further, the deposition rate constantly decreases during bath life and a deposition rate too low finally terminates the life time of an electroless palladium plating bath. This is due to the catalytic effect of already deposited palladium and the autocatalytic deposition mechanism. Usually, changing the temperature of an electroless palladium plating bath is used for regulating the deposition rate and duration of bath life. Increasing the bath temperature also increases the deposition rate. But operating the bath at a higher temperature simultaneously increases the risk of destabilising the bath.
The stability of such a plating bath means that the plating bath is stable against decomposition, i.e. the undesired precipitation of metallic palladium in the plating bath itself. Accordingly, destabilising an electroless palladium plating bath in turn shortens the bath life. Due to the high price of palladium early discarding of an electroless palladium plating bath is undesired for economic reasons as well.